1. Field of the Invention
The present invention relates to a plastic-sealed IC device packed in a plastic package resistant to thermal cracking.
2. Description of the Prior Art
A plastic-sealed IC device formed by sealing an IC chip mounted on a lead frame with a resin, such as epoxy resin, can be manufactured at a lower manufacturing cost as compared with an equivalent ceramic-sealed IC device, and is capable of being manufactured through mass production processes. Accordingly, in resent years, plastic-sealed IC devices have been applied to the production of DIPs (dual-in-line packages).
A prior art relating to a plastic-sealed IC device is disclosed, for example, in U.S. Pat. No. 3,611,061.
Recently, flat plastic-sealed IC devices having a plurality of terminal pins extending from the circumference of the package have been put to practical use, with the enhancement of the degree of integration of IC devices. A process of manufacture of such a flat plastic-sealed IC device is disclosed in U.S. Pat. No. 3,444,440. This flat plasticsealed IC device, indicated at 10 in FIG. 1, in general is mounted on a printed circuit board 12 as shown in FIG. 1.
The pins 14 of the IC device are connected fixedly by soldering to the conductors 16 of the printed circuit board 12.
Recently, the soldering process has been automated for mass production, in which a printed circuit board mounted with IC devices is dipped directly into molten solder of around 200.degree. to 260.degree. C. temperature or the printed circuit board is heated through infrared heating.
However, a crack 18, as illustrated in FIG. 1, often develops in the package 20 of a flat plastic-sealed IC device when the flat plastic-sealed IC device is subjected to the soldering process in which the entirety of the flat plasticsealed IC device is heated.
The estimated causes of such cracking are as follows. Generally, a resin molding contains a mold lubricant to facilitate the extraction of the same from the mold after solidification, and hence a gap is formed inevitably between the resin molding and the lead frame. Consequently, moisture permeates the IC chip mounting area 22 along the surfaces of the pins of the lead frame.
Furthermore, since resins for sealing an IC chip, in general, are water-permeable, moisture is liable to accumulate around the IC chip mounting area. As shown in FIG. 7, for instance, it is confirmed that moisture penetrates into the vicinity of the IC chip 24 even when a plastic-sealed IC device is placed merely in the atmosphere for about 100 hours.
On the other hand, the strength of the resin molding is deteriorated when the resin molding is heated to a temperature corresponding to the fusing temperature of the solder.
It is inferred that when a plastic-sealed IC device containing moisture is heated in the soldering process, the moisture accumulating in the vicinity of the IC chip mounting area 22 evaporates and expands to produce the crack 18 in the plastic package.
The crack in the plastic package in the soldering process spoils the appearance of the plastic-sealed IC device and, further, causes the breakage of conductors 26 interconnecting the IC chip and the lead frame and the corrosion of aluminium wiring formed over the surface of the IC chip due to moisture permeation.